1. Field of the Invention
The present invention relates to a circuit board and a method of manufacturing the same, and more specifically, to a circuit board, which allows a large electric current to flow through a circuit pattern therein, and a method of manufacturing the same.
2. Description of the Related Art
In general, use of electronic circuits has been rapidly expanded to such fields as industry machines and automobile parts. In particular, a composite circuit board has been widely used in an inverter circuit, a control circuit of a servo motor, a power supply unit, or the like. In the composite circuit board, a thin circuit pattern for a small current (for a signal) and a thick circuit pattern for a large current are formed in one insulating board. Various circuit boards having such composite circuits have been known.
FIG. 8 is a cross sectional view showing a large current circuit board according to a conventional technology. As shown in FIG. 8, a large current circuit board 60 includes an insulating board 61 made of glass epoxy material or the like; conductors 62,63 formed with copper used therein; a control circuit conductor 64 made of copper by etching both sides of the insulating board 61 or the like; and a through hole part 65 which penetrates the insulating board 61 and the conductors 62,63. Interfaces 67 between the through hole part 65 and the conductors 62,63 are copper-plated, respectively, thus forming a predetermined current conductor path through the conductors 62,63,64. Voltages having opposite polarities are applied to the conductors 62,63. In the large current circuit board 60, an ordinary conductor has a thickness of about 35 μm.
FIG. 9 and FIG. 10 are a cross sectional view and a perspective view, respectively, each showing another large current circuit board according to a conventional technology. As shown in FIG. 9 and FIG. 10, a large current circuit board 70 is formed such that different burred terminals 75 are electrically connected on an upper surface of the board 70 via a thick conductor 74 (see, for example, Japanese Laid-Open Patent Application, Publication No. H10-56244, Claim 1, FIG. 1).
Namely, the large current circuit board has a signal conductor 72 and patterned conductors 73 formed by attaching copper foil to a surface 71a of the insulating board 71 made of glass epoxy material or the like, and etching the copper foil into a predetermined pattern. The thick conductor 74 is formed by cutting a copper sheet into a predetermined pattern, and is soldered to the pattered conductor 73, which is etched in a shape corresponding to the large current thick conductor 74.
In the large current circuit board 70, the burred terminal 75 burred in a cylindrical shape is inserted into the through hole part 76, which penetrates the insulating board 71. Then an end 75a of the burred terminal 75 is soldered to a patterned conductor 77, which is formed on an underside 71b of the insulating board 71. As shown in FIG. 10, when a plurality of different burred terminals 75 are electrically connected, a plurality of thick conductors 74 each in a rod shape with an insulated surface thereof are disposed one above another. This means that the large current circuit board 70 has a larger volume in a height direction thereof, as a circuit thereof becomes more complicated. In the large current circuit board 70, an ordinary conductor has a thickness of about 210 to 500 μm.
The large current circuit board 60 shown in FIG. 8 is not suitable, however, for flowing a large current therethrough, because an area of the interfaces 67 between the through hole part 65 and the conductors 62,63 are substantially small. Nonetheless, reduction of a combined inductance (a surge reduction) can be expected, since the conductors 62,63 in the insulating board 61 are provided close to each other. To flow a large current through the large current circuit board 60 shown in FIG. 8, it is necessary to provide a large number of the through hole parts 65, to thereby increase a total electrical contact area of the interfaces 67. However, a more complicated manufacturing process is required to provide the large number of the through hole parts 65.
As compared to the large current circuit board 60, the large current circuit board 70 shown in FIG. 9 and FIG. 10 is suitable for flowing a large current therethrough. However, the large current circuit board 70 requires a longer time in forming and placing the thick conductors 74 therein, because the thick conductors 74 have to be placed between a plurality of the through hole parts 76 for each current path. Further, there may be a region in the large current circuit board 70, in which a plurality of the thick conductors 74 are multilayered in the height direction. As a result, an area for placing other components inevitably becomes smaller, and a thickness of the board becomes larger. Moreover, reduction of a combined inductance (a surge reduction) cannot be so much expected in the large current circuit board 70 as in the large current circuit board 60 shown in FIG. 8, because a distance between the opposing patterned conductors 73,77 is larger in the large current circuit board 70.
The present invention has been made in light of the above-mentioned problems. The present invention provides a circuit board capable of flowing a large current therethrough with a relatively simple configuration and without increasing a size thereof in the thickness direction. The present invention also provides a method of manufacturing the circuit board.